The present invention generally relates to a satellite communications network and more specifically to a satellite communications network that supports parallel, independent communications links with separate terminals over separate modulation channels of a multi-dimensional modulator.
Satellite communication systems have been proposed for relaying communications signals between one or more terminals and a ground station or earth station. The terminals (mobile or fixed) may be located at various locations about the world with multiple terminals located in a common field of view of one satellite. Quite often, a single beam spot may have several hundred users located therein. The satellite typically includes one or more antenna arrays that define beam spots or footprints on the earth's surface. Each beam spot defines a coverage area which may be several hundred miles in diameter. Terminals in the coverage area of a beam spot communicate with the satellite over predetermined uplink and downlink frequency spectra associated with the beam spot. Ground stations communicate with the satellite over predetermined feeder uplink and downlink frequency spectra.
Satellite systems have been proposed that utilize different types of access schemes to maximize the number of users that may communicate with a satellite while in a single beam spot's coverage area. For instance, access schemes that have been proposed include time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA) and the like. Optionally, multiple carrier frequencies may be used within a single beam in a hybrid TDMA and FDMA type system.
In a system supporting a TDMA format, the satellite supports a beam having at least one carrier frequency received by all user terminals in the beam's coverage area that are assigned to the carrier frequency. The carrier frequency is temporally divided into a sequence of consecutive, adjacent time slots. Each time slot holds cells or packets of communications data designated for a particular user terminal. The time slots are organized into master frames. Each given master frame comprises a predetermined number of time slots, during which data packets intended for a particular user terminal are modulated onto the carrier frequency. Each data packet includes addressing information identifying the destination user terminal. All user terminals in a single beam's coverage area and assigned to a common carrier frequency receive all data transmitted over that carrier frequency. Each user terminal then identifies the data packets designated for the corresponding user terminal based upon the addressing information therein.
Communications satellites typically receive communications data streams over separate terminal uplinks and combine, in the satellite, the data into a common feeder downlink designated for a corresponding ground station. The composite signal transmitted to the ground station is conveyed over a feeder downlink. In a feeder uplink, a ground station transmits data cells arranged according to a TDMA format. The data cells in the feeder uplink are modulated upon a feeder uplink carrier frequency. Exemplary modulation techniques include quadrature phase shift keying (QPSK), 8 phase shift keying (8PSK), 16 quadrature amplitude modulation (16-QAM), 32-QAM modulating, and the like.
The satellite may merely represent a “bent-pipe” type satellite, in which the data received over the feeder uplink is simply translated in frequency and transmitted over one or more corresponding beams. Bent pipe satellites offer limited on-board processing. Alternatively, the satellite may be more intelligent than a bent-pipe type satellite. For instance, the satellite may perform on-board processing including, inter alia, decoding data, redirecting data to one of multiple destinations, directly interconnecting source and destination terminals and the like.
A feeder uplink may further be divided into multiple frequency subbands, each frequency subband of which is uniquely associated in a one-to-one relation with a beam defined by the satellite's antenna array. An exemplary bent-pipe type satellite may merely translate the carrier frequency associated with a particular frequency subband to a carrier frequency assigned to a corresponding beam. The satellite may then transmit all of the received data modulated over the carrier frequency of the beam associated with the frequency subband of the feeder uplink. The return communications link from a user terminal to a ground station is established in a similar manner by modulating communications data over a carrier frequency assigned to the terminal-to-satellite uplink and then frequency translated by the satellite and relayed over a corresponding frequency subband in a feeder downlink.
The communications links established between user terminals and satellites vary substantially in quality. Signal quality is dependent in part upon signal strength. Signal strength varies depending upon the geographic position at which a user terminal is located. Signal strength also varies depending upon the user terminal's relative position within the coverage area of a particular beam spot. The signal characteristics associated with a particular beam spot are experienced unevenly across the beam spot. Typically, greater signal strength is experienced at the center of a beam spot, while weaker signal strength is experienced at the outer edges of a beam spot. Hence, terminals located at the outer edge of a coverage area of a beam spot may experience a weaker communications link with the satellite in both the uplink and downlink, as compared to the link maintained between a satellite and a user terminal located at the center of the coverage area. In addition, terminals located in geographic regions of the world that experience a substantial amount of cloud cover or rainfall may experience a weaker connection with the satellite than user terminals located in areas with a clear sky.
Satellite systems have been proposed that attempt to address the problems experienced by weak signal strength. For example, systems have been proposed for encoding the communications data with an error correction code. The error correction code enables the terminal and ground station to identify errors within the communications data stream and either correct each error or retransmit the data. Exemplary encoder techniques include convolutional encoders, block encoders and the like.
Each terminal and ground station includes a modulator for placing the data on the carrier frequency. An exemplary modulator is a QPSK modulator. A QPSK modulator includes two modulation channels, namely an I channel and Q channel. A QPSK modulator represents a two-dimensional modulator as it includes two modulation channels. An 8PSK modulator includes three modulation channels and represents a three-dimensional modulator. Similarly, a 16-QAM modulator includes four channels and thus represents a four-dimensional modulator, while a 32-QAM modulator includes five modulation channels and represents a five-dimensional modulator.
User terminals and ground stations have been proposed which use QPSK modulators having a first encoder connected to the I channel and a second encoder connected to the Q channel. A single communications data stream to/from one terminal is divided between the first and second encoders such as through multiplexing by which alternate bits of the single data stream are alternately provided to the first and second encoders. The first and second encoders then encode corresponding portions of the single data stream at a common error correction coding rate and provide the encoded partial data streams to the QPSK modulator. The QPSK modulator then modulates upon the carrier frequency half of the encoded data stream on the I channel and half of the encoded data stream on the Q channel.
The demodulator at the receiving terminal or ground station demodulates the I channel half of the encoded data stream and the Q channel the other half of the encoded data stream. Corresponding decoders are attached to the I and Q channel outputs of the demodulator and decode corresponding halves of the data stream. The partial data streams output from the first and second decoders are then recombined in an alternating manner to form a single data stream. The foregoing structure is repeated in the forward communications link from the ground station to the terminal and reverse communications link from the terminal to the ground station.
Optionally, a single encoder may be used, with the encoded data stream output from the encoder being evenly divided between the I channel and Q channel of the modulator. Further, a single decoder may be used, whereby the I and Q channel outputs from the demodulator are first combined by multiplexing and then passed through the single demodulator.
However, proposed systems have certain drawbacks. First, proposed systems require each user terminal to demodulate and process (e.g., encode and decode) both the partial (half) data streams carried over the I and Q channels in order to form each data packet from a received signal. Requiring each user terminal to incorporate both I and Q channel data processing hardware and software increases the cost and complexity of each user terminal. A second disadvantage of proposed systems is that all user terminals in a given beam spot that are assigned to a common carrier frequency must implement a common amount of error correction encoding/decoding in order to encode/decode data packets addressed thereto and transmitted therefrom. The encoding scheme designated for a given carrier frequency, in proposed systems, is designed based on the “worst case scenario”, in which the weakest signal strength is estimated for the entire beam spot. For instance, a system may be designed based on the assumption that a user at the outer edge of a beam spot requires a one-half encoder rate; that is for every bit of data provided to the encoder, two encoded bits are generated for modulation and transmission. Hence, a system requiring a 200 Megabit per second information data rate would require a 400 Megabit per second transmission data rate to account for the encoded data.
However, while users at the edge of a beam spot may require a one-half encoder rate to afford sufficient error correction to account for signal weakness, users at the center of the same beam spot experience a much stronger signal and therefore require significantly less encoding, such as a two-thirds or three-fourths encoder rate. Conventional systems require a common amount of encoding for all users assigned to a common carrier frequency. Accordingly, a system designed to afford a one-half encoder rate for users at the edge of a beam spot unnecessarily requires users at the center of the beam spot to use a one-half encoder rate, thereby mandating unneeded error correction encoding and decoding. Requiring excessive error correction decoding unnecessarily wastes system bandwidth and unduly limits the information data rate that may be afforded to users at the center of the beam spot and users having strong signal connections with the satellite.
A need remains for an improved communications satellite system. It is an object of the present invention to meet this need.